例句：
When exactly did the incident occur?
这一事件究竟是什么时候发生的.？
These snails do not occur on low-lying coral islands.
这些蜗牛不会出现在低洼的珊瑚岛上。
Rattlesnakes occur in the warmer, drier parts of North America.
响尾蛇出现在北美温暖干燥的地区。
Nitrates are chemicals that occur naturally in water and the soil.
硝酸盐是水和土壤里天然存在的化学物质。